This invention relates to novel processes for the production of coated glass during a float glass production process.
The float glass process, developed by Pilkington in 1952, is now the world standard for high quality glass production. Float glass is often processed further before being fitted into buildings and vehicles.
The process is used to make glass as thin as 0.4 mm and as thick as 25 mm. A ‘batch’ of precisely mixed raw materials is melted in a furnace. Molten glass, at approximately 1000° C., is poured continuously from the furnace onto a shallow bath of molten tin in a chemically controlled atmosphere. The glass floats on the tin, spreads out and forms a level surface. Thickness is controlled by the speed at which the solidifying glass ribbon is drawn off from the bath. The ribbon of glass which is produced is passed to a lehr in which it cools under controlled conditions. After annealing the glass emerges as a ‘fire’ polished product with virtually parallel surfaces.
The heat and light transmission properties of the glass may be varied by depositing a transparent coating upon at least one surface of the glass ribbon. These coatings may be deposited during the float glass production process. The processes which are used to deposit such coatings are commonly termed on line coating processes. Such processes are well established in commercial production and may be carried out using atmospheric pressure chemical vapour deposition processes (hereinafter for convenience APCVD processes). In an APCVD process a fluid mixture is directed onto the surface of the glass ribbon at a point where the ribbon is at an elevated temperature and the heat of the glass causes the reaction of the components of the fluid mixture and the deposition of a coating upon the surface of the glass ribbon. Such processes are particularly useful in the float bath. The float bath is maintained under a reducing atmosphere in order to avoid oxidation of the tin and is maintained at substantially atmospheric pressure. The glass is at a high temperature typically from 750° C. to 400° C.
Such processes may be carried out using known methods and apparatus such as those which are described in EP 785868. The apparatus typically comprises one or more distributor beams which extend transversely across the float bath section. The processes are designed to coat the entire width of the glass ribbon and the distributor beams are typically represented as extending across the entire width of the ribbon. However in practice the distributor beams do not extend to the edges of the ribbon because of the potential for the components of the fluid mixture to react with and/or contaminate the tin in the float bath. The extremities of the ribbon are left uncoated and are cut off after the ribbon leaves the annealing lehr.
One problem which is commonly encountered in a float glass production process is the breakage of the glass ribbon as it passes through the annealing lehr. These breakages are unpredictable events which reduce the yield of usable glass and which detract from the economics of the process. They occur during the production of uncoated glass possibly because the extremities of the ribbon, which are often termed the knurl, are thinner than the centre of the ribbon which leads to thermal and mechanical stresses in the ribbon. The problem of breakages in the ribbon is exacerbated when a coated glass ribbon is produced. The rate of heat loss in the ribbon is greater in the uncoated edges of the ribbon than in the coated section of the ribbon. This results in a temperature differential between these two portions of the ribbon as it passes through the annealing lehr which causes a thermal stress which can result in breaking of the ribbon which is undesirable.
Attempts have been made to reduce the number of breakages by providing heating means to raise the temperature of the uncoated edges of the ribbon. These attempts have not been entirely successful particularly when the coated section of the glass ribbon has a thickness of greater than 4 mm.